During the past several years, substantial improvement has been made in three dimensional (3D) seismic surveys to better locate and define the boundaries of underground hydrocarbon producing formations. During this same time period, substantial improvements have also been made in directional drilling and horizontal well completion techniques. As a result, many current well completions often include more than one wellbore or borehole. For example, a first, generally vertical wellbore may be initially drilled within or adjacent to one or more hydrocarbon producing formations. Multiple wellbores may then be drilled extending generally laterally from the first wellbore to respective locations or "targets" selected to optimize production from the associated hydrocarbon producing formation or formations. Such well completions are often referred to as multilateral wells.
A typical multilateral well completion may include a first or primary wellbore defined in part by a casing string and a layer of cement disposed between the exterior of the casing string and the inside diameter of the primary wellbore. The primary wellbore will generally extend from the well surface to a desired downhole location. The primary wellbore may be substantially vertical or may be slanted depending upon the location of the associated wellhead and downhole hydrocarbon producing formation or formations. For some well completions the primary wellbore may include a substantially vertical portion extending from the well surface and one or more portions which are drilled at a selected angle or angles relative to the vertical portion. The primary wellbore may also include one or more portions having an up dip or a down dip.
Directional drilling equipment and techniques may be used to form one or more exits or windows from the primary wellbore through the associated casing string and layer of cement at respective downhole locations selected for drilling secondary wellbores extending from the primary wellbore. The location and orientation of each window in the primary wellbore and the length, diameter and orientation of respective secondary wellbores relative to the primary wellbore are generally selected to optimize production from the associated hydrocarbon producing formation or formations. For many well completions such as deep offshore wells, multiple secondary wellbores will be drilled from each primary wellbore in an effort to optimize hydrocarbon production while minimizing overall drilling and well completion costs. Selective isolation and/or reentry into each of the secondary wellbores is often necessary to further optimize production from the associated hydrocarbon producing formation or formations.
A typical multilateral well completion may have one or more production tubing strings disposed within the casing string of the primary wellbore. The production tubing string or strings will have a generally uniform inside diameter extending from the well surface to a selected downhole location. A reentry window assembly, sometimes referred to as a lateral reentry window may be installed within each production tubing string at a downhole location corresponding with the location at which a secondary wellbore intersects the primary wellbore. For example, a multilateral well completion may include a primary wellbore extending substantially vertically from the well surface and three secondary wellbores extending substantially laterally from the primary wellbore at respective first, second and third downhole locations. A production tubing string with three window assemblies may be installed within the casing string of the primary wellbore using conventional well completion techniques such that each window assembly is disposed adjacent to and aligned with a respective window in the primary wellbore and associated lateral or secondary wellbore.
For some multilateral well completions, a primary wellbore may be drilled and a casing string disposed therein using conventional well drilling and completion techniques. One or more casing nipples may be installed within the casing string proximate respective downhole locations from which respective secondary wellbores will be drilled. Various types of locking mandrels and/or latches may be used to install equipment such as a whipstock in the respective casing nipple for use in forming an associated secondary wellbore extending from the primary wellbore and/or to allow reentry from the primary wellbore to the secondary wellbore.
For other multilateral well completions, a primary wellbore may be drilled and a casing string disposed therein using conventional well drilling and completion techniques. A whipstock packer may then be installed within the casing string of the primary wellbore proximate a desired location for drilling a secondary wellbore extending therefrom. When drilling and completion of the secondary wellbore has been completed, the associated whipstock packer may be removed from the primary wellbore to allow access through the primary wellbore to other downhole hydrocarbon producing formations. The same or another whipstock packer may be installed in the primary wellbore at another location for use in forming an additional secondary wellbore extending therefrom. Whipstock packers may also be used to reenter a secondary wellbore extending from primary wellbore. However, production tubing strings and other downhole equipment must often be removed from the primary wellbore before a whipstock packer can be placed therein. After removal of the associated whipstock packer, it may often be difficult to locate a window in the casing and to align equipment with the window for reentry from the primary wellbore into the respective secondary wellbore on a reliable, consistent basis.